Towards quantitative catalytic lignin depolymerization

The products of base-catalyzed liquid-phase hydrolysis of lignin depend markedly on the operating conditions. By varying temperature, pressure, catalyst concentration, and residence time, the yield of monomers and oligomers from depolymerized lignin can be adjusted. It is shown that monomers of phenolic derivatives are the only primary products of base-catalyzed hydrolysis and that oligomers form as secondary products. Oligomerization and polymerization of these highly reactive products, however, limit the amount of obtainable product oil containing low-molecular-weight phenolic products. Therefore, inhibition of concurrent oligomerization and polymerization reactions during hydrothermal lignin depolymerization is important to enhance product yields. Applying boric acid as a capping agent to suppress addition and condensation reactions of initially formed products is presented as a successful approach in this direction. Combination of base-catalyzed lignin hydrolysis with addition of boric acid protecting agent shifts the product distribution to lower molecular weight compounds and increases product yields beyond 85%.     

Spectroscopic and electronic structure studies of a dimethyl sulfoxide reductase catalytic intermediate: implications for electron- and atom-transfer reactivity

The electronic structure of a genuine paramagnetic des-oxo Mo(V) catalytic intermediate in the reaction of dimethyl sulfoxide reductase (DMSOR) with (CH(3))(3)NO has been probed by electron paramagnetic resonance (EPR), electronic absorption, and magnetic circular dichroism (MCD) spectroscopies. EPR spectroscopy reveals rhombic g- and A-tensors that indicate a low-symmetry geometry for this intermediate and a singly occupied molecular orbital that is dominantly metal centered. The excited-state spectroscopic data were interpreted in the context of electronic structure calculations, and this has resulted in a full assignment of the observed MCD and electronic absorption bands, a detailed understanding of the metal-ligand bonding scheme, and an evaluation of the Mo(V) coordination geometry and Mo(V)-S(dithiolene) covalency as it pertains to the stability of the intermediate and electron-transfer regeneration. Finally, the relationship between des-oxo Mo(V) and des-oxo Mo(IV) geometric and electronic structures is discussed relative to the reaction coordinate in members of the DMSOR enzyme family.     

Tame combing and almost convexity conditions

We give the first examples of groups which admit a tame combing with linear radial tameness function with respect to any choice of finite presentation, but which are not minimally almost convex on a standard generating set. Namely, we explicitly construct such combings for Thompson??s group F and the Baumslag?CSolitar groups BS(1, p) with p ??? 3. In order to make this construction for Thompson??s group F, we significantly expand the understanding of the Cayley complex of this group with respect to the standard finite presentation. In particular we describe a quasigeodesic set of normal forms and combinatorially classify the arrangements of 2-cells adjacent to edges that do not lie on normal form paths.     

A computational study of tert-butylbenzenium ions

A computational study of tert-butylbenzenium ions has been performed. Structures and energies of the various isomers and the transition states for their interconversions have been determined. The existence of a stable π-electron complex (called A1) between a tert-butyl cation and a benzene molecule has been confirmed. Other minimum points on the potential energy surface corresponding to π-complexes were found, but the barriers for transforming these complexes into the stable π-electron complex A1 are so low that the transformation into the stable structure can take place at all temperatures. The structures were evaluated at the DFT-B3LYP level of theory. The energies were evaluated with the Gaussian-3 (G3B3) and CBS (CBS-QB3) composite methodologies. A discussion of published experimental data in view of the computational results is given. It is pointed out that it should be possible to show the presence of the π-complex by IR spectroscopy. The computations show that the π-complex has a very strong IR band in an otherwise empty region.     

The micromechanics of three‐dimensional collagen‐I gels

We study the micromechanics of collagen‐I gel with the goal of bridging the gap between theory and experiment in the study of biopolymer networks. Three‐dimensional images of fluorescently labeled collagen are obtained by confocal microscopy, and the network geometry is extracted using a 3D network skeletonization algorithm. Each fiber is modeled as an elastic beam that resists stretching and bending, and each crosslink is modeled as torsional spring. The stress–strain curves of networks at three different densities are compared with rheology measurements. The model shows good agreement with experiment, confirming that strain stiffening of collagen can be explained entirely by geometric realignment of the network, as opposed to entropic stiffening of individual fibers. The model also suggests that at small strains, crosslink deformation is the main contributer to network stiffness, whereas at large strains, fiber stretching dominates. As this modeling effort uses networks with realistic geometries, this analysis can ultimately serve as a tool for understanding how the mechanics of fibers and crosslinks at the microscopic level produce the macroscopic properties of the network. © 2010 Wiley Periodicals, Inc. Complexity 16: 22‐28, 2011     

A check for consistency between positron-atomic hydrogen total,positronium formation,and ionization cross section measurements

We have recently measured total cross sections for 5 to 302 eV positrons scattered by atomic hydrogen using a beam transmission technique. A check for consistency is made between these measurements and prior measurements of positronium formation and ionization cross sections by making use of available theoretical calculations of elastic and excitation cross sections for positron-atomic hydrogen scattering.     

Mobile und immobile Probensammlung mit Hilfe von Bienen und Birken

Summary The analysis of birch sap and honey has been used to predicase the metal-loading over a large area. The distribution of lead and cadmium in Münster and its surroundings has been investigated. In general, the metal concentrations in birch sap and honey were found to depend on the pH-value and the metal content of the soil. In the case of lead, the higher concentration at the busy highway areas can be easily observed. It is evident that a noise protection wall is also a protection against lead emission from the exhaust of the automobiles in the region of Nienberge, a suburb of the city of Münster.